The purpose of this study is to explore the feasibility of time-dependent acoustic impedance measurement by Scanning Acoustic Microscopy (SAM) for analyzing the sodium diffusion. The purpose is motivated by the fact that sodium monitoring is challenging and still in the area of exploratory analysis despite its biological importance. To our knowledge, this is the first study in which sodium diffusion has been investigated by time-dependent acoustic impedance measurements provided by SAM. We first tested the idea in an agarose phantom as a proof-of-concept. Accordingly, we designed the agarose phantom which initially contains a well of sodium chloride (NaCl) solution moving radially into the phantom. By using NaCl diffusion in the phantom, we obtained two-dimensional (2D) acoustic impedance (Z) maps over time through SAM operating with 80 MHz ultrasonic transducer having a lateral resolution of 20 mu m. A linear correlation between the changes in the concentration profile of the phantom and its acoustic impedance was introduced. Analysis of experimental data proved that spatially changing acoustic impedance could be ascribed to the diffusion process and produced a diffusion coefficient in the order of 10(-5)cm(2)/s which matches well with the literature. Our results showed that SAM could monitor the time-dependent alterations in acoustic impedance resulting from the diffusion of sodium inside the agarose phantom. With this study, SAM shows a promise as a monitoring tool not only to obtain static images but also to perform dynamic investigations of sodium ions with the advantages of providing images in micrometer resolution with a scanning time no longer than 2 min for an image area of 4.8 mm x 4.8 mm.